Q. What is the circle-cross stamp on some of my ammo?
The circle-cross Å is the NATO symbol. It indicates that the ammo was loaded in a NATO-approved facility and meets the NATO specifications for that round. Note that NATO specifications are not the same as US military specifications and that many NATO-approved rounds do not meet US military specs. US military specs (such as M193 and M855) have additional requirements, such as minimum velocities, that the NATO specs (like SS-109) don't have.
Fact: There are some exceptions to this rule. For example, recent Lake City and Winchester M193 is loaded in cases marked with the NATO circle-cross. This is done simply to save money by having one production run of cases instead of two. M193 was never adopted by NATO; by the time NATO decided to standardize on 5.56mm, the SS-109/M855 ammo was available, and was adopted as the standard. M193 is still "Mil-Spec," it just isn't "NATO" spec.